1. Field of the Invention
The present invention relates to an automatic speed reducing ratio-switching apparatus capable of automatically switching the speed reducing ratio transmitted from an output shaft to a displacement member of an actuator by using a planetary gear mechanism when a load, which exceeds a predetermined torque, is applied.
2. Description of the Related Art
Those to which the speed reducing ratio-switching mechanism has been hitherto applied include, for example, machine systems such as construction machines. In such a machine system, an electric cylinder is used as an actuator for an expansion/contraction operation system for driving a link mechanism.
In such an electric cylinder, a rotary shaft is connected to an input section of an electric motor in a casing, and a screw shaft is arranged in the rotary shaft. The screw shaft is screwed to a nut member which is rotatably supported in the casing. Two pairs of planetary gear mechanisms, which have different speed reducing ratios, are provided between the rotary shaft and the nut member. Each of the planetary gear mechanisms comprises a sun gear, and planet gears meshed with each sun gear and an internal gear provided inside the cylindrical casing to perform the planetary motion. Each of the sun gears is connected to the rotary shaft via a one-way clutch in which the direction of engagement is different to be the forward direction and the reverse direction. A planetary support shaft, which rotatably supports the planet gears of each of the planetary gear mechanism, is connected to the nut member.
In the electric cylinder, when the electric motor is driven and rotated in the forward direction, then the rotary shaft is also rotated in the forward direction, the nut member is rotated in the forward direction by the aid of the planetary gear mechanism having a small speed reducing ratio, and thus the screw shaft is moved in an expandable manner. On the other hand, when the electric motor is driven and rotated in the reverse direction, then the rotary shaft is also rotated in the reverse direction, the nut member is rotated in the reverse direction by the aid of the planetary gear mechanism having a large speed reducing ratio, and thus the screw shaft is moved in a contractible manner (see Japanese Laid-Open Patent Publication No. 2003-184982).
However, in the electric cylinder as described above, it is necessary to use the two types of planetary gear mechanisms having different speed reducing ratios in order to allow the screw shaft to perform the expanding operation and the contracting operation. In this case, the number of parts is increased, and the entire electric cylinder becomes large in size. In the planetary gear mechanism of the electric cylinder as described above, the expanding operation is performed at low speed with large thrust force, while the contracting operation is performed at high speed with small thrust force, irrelevant to the magnitude of the load torque applied to the electric cylinder. Therefore, the movement speed of the screw shaft cannot be increased to a high speed for the expanding operation even when the load torque applied to the electric cylinder is small.